A number of processes have have been reported for the preparation of aromatic mono- and polyisocyanates by the vapor or solvent phase decomposition of substituted ureas.
The vapor phase production of aromatic isocyanates from symmetrical bis aryl ureas in the presence of hydrogen chloride,phosphorus pentoxide or zinc chloride was described by A. Hofmann in the Proc. Royal Soc., London, Vol. IX, p. 274 (1858). By heating a mixture of diphenyl urea with phosphorus pentoxide, zinc chloride or gaseous HCl. Hofmann distilled phenyl isocyanate ovehead. No details of the experimental procedure are presented and yield of isocyanate not given.
A. Hofmann, Chemisch Berichte, Vol. 3, pp. 653-658 (1870) described heating diphenyl urea in the presence of phosphoric acid giving yields too small to be considered for the preparation of the isocyanate.
Subsequent work by Iwakura and Nagakubo reported in the Bulletin Tokyo Inst. Technol., Vol. 13, p.25 (1950) and Chemical Abstracts, Vol.44, p. 3924e (1950) describes the preparation of an aromatic isocyanate (p-ethoxyphenylisocyanate) by heating a solution of bis aryl urea such as bis (p-ethoxyphenyl) urea in the presence of hydrogen chloride gas.
The vapor phase decomposition of bis aryl ureas at 350.degree. C. and higher temperatures has been described by W.D. Bennet et al, Journ. Am. Chem. Soc., Vol. 75, p.2101 (1952) and Slocombe et al in U.S. Pat. No. 2,773,086, Dec. 4, 1956 in the presence of gaseous HCl as a promoter. Yields are reported in the 60 to 70% range for the vapor phase reaction and only a 5% yield for liquid phase reaction. A carbamoyl chloride intermediate is formed.
The liquid phase decomposition of trisubstituted ureas to isocyanates has been described by van Landeghem et al, French Pat. No. 1,473,821, Feb. 13, 1967; C. J. Hearsey, U.S. Pat. No. 3,898,259, Aug. 5, 1975 and Rosenthal et al in the U.S. Pat. No. 3,936,484, Feb. 3, 1976. van Landeghem shows thermal decomposition of trisubstituted ureas in an organic solvent having specified dielectric constants at 140.degree. to 170.degree. C. with long reaction times of from 6 to 10 hours and modest yields of 60 to 75%. A variety of catalysts are shown but not exemplified or claimed, and include metal salts, such as acetates, stearates, and linoleates of manganese, zinc, cobalt, chromium and vanadium, tertiary amine bases, such as aliphatic, cycloaliphatic, aromatic and mixed tertiary amines, aliphatic heterocyclic amines such as N-methylpiperidine or N, N'-dimethylpiperidine as well as aromatic heterocyclic amines such as pyridine and pyrimidine. Other nitrogen compounds such as imidazole are indicated as being suitable. However, under the reaction conditions described tertiary amines as shown by van Landeghem do not catalyze urea decomposition.
Rosenthal et al U.S. Pat. No. 3,936,484 discloses the thermal decomposition of di- and tri-substituted ureas to isocyanates at temperatures above 230.degree. C. in a solvent with short residence times and isocyanate yields of from 60 to 80%.
The Hearsey U.S. Pat. No. 3,898,259 describes the introduction of gaseous hydrogen chloride into the liquid phase urea decomposition reaction to give reduced reaction times with isocyanate yields of from 80-90%. An excess of gaseous HCl is employed at temperatures of from 100.degree. to 200.degree. C. and a by-product carbamoyl chloride intermediate formed.
A. Hentschel et al U.S. Pat. No. 4,223,145, Sept. 16, 1980 discloses the formation of an HCl adduct of a tri-substituted urea using at most, a 10% excess of HCl. This adduct is then decomposed in a closed system at from 80.degree.-180.degree. C.
Applicants have found that organic sulfonic acids are excellent promoters for the thermal decomposition of tri-substituted ureas to the corresponding isocyanates such as tolylene diisocyanates at relatively mild reaction temperatures and short residence times in an organic solvent. The reaction gives the corresponding isocyanate in almost quantitative yield.